C15 Laves phase intermetallic ${\mathrm{CeFe}}_2$ show reduced lattice constant and lower Curie temperature due to a strong $4f\text{−}3d$ hybridization. This hybridization is also believed to cause a ferromagnetic-to-antiferromagnetic phase transition in ${\mathrm{CeFe}}_2$ under hydrostatic pressure. In this work, we use first-principles calculations to study the magnetic stability of ${\mathrm{CeFe}}_2$ under high pressure. With LDA functional, we predict a ferromagnetic-to-antiferromagnetic phase transition of ${\mathrm{CeFe}}_2$ at 42.5 GPa. We also study how the projected density of states of $\mathrm{Ce}\text{−}4f$ and $\mathrm{Fe}\text{−}3d$ evolves with pressure. For both ferromagnetic and antiferromagnetic phases, we find an enhanced hybridization between $\mathrm{Ce}\text{−}4f$ and $\mathrm{Fe}\text{−}3d$ states which favors antiferromagnetism with the increase of pressure.

The electron spin polarization on half-metallic double perovskites is usually conditioned to the ordered rock-salt arrangement of the transition-metal ions along the lattice. In this work, we investigate a polycrystalline sample of the ${\mathrm{Ca}}_{1.5}{\mathrm{La}}_{0.5}{\mathrm{MnRuO}}_6$ compound by employing x-ray powder diffraction, high-resolution transmission electron microscopy, x-ray absorption and magnetic circular dichroism at the Mn $L_{2,3}$ and Ru $M_{2,3}$ edges, magnetometry, electrical transport, and first-principles calculations in order to show that this is a fully disordered material exhibiting near-room-temperature ferrimagnetism and half-metallic conductivity, with significant intergrain tunneling magnetoresistance. These unprecedented results are compared to those of archetypical ordered double perovskites, and discussed in terms of the Mn and Ru valences and their orbital hybridization.

We show that the high-temperature expansion of the free energy and arbitrary imaginary-time-ordered connected correlation functions of quantum spin systems can be recursively obtained from the exact renormalization group flow equation for the generating functional of connected spin correlation functions derived by Krieg and Kopietz [Phys. Rev. B 99, 060403(R) (2019)]. Our recursive algorithm can be explicitly written in closed form including all combinatorial factors. We use our method to estimate critical temperatures of Heisenberg magnets from low-order truncations of the inverse spin susceptibility in the static limit. We also calculate the connected correlation function involving three different spin components (chiral nonlinear susceptibility) of quantum Heisenberg magnets up to second order in the exchange couplings.

Ferromagnetic graphene is particularly promising for spintronics devices applications, in which Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction is widely considered to play the crucial role in the magnetic coupling between the adjacent localized spins. Recent theoretical studies show that integrating electron transfer effect into the typical RKKY interaction can effectively tune the decay rate and the oscillation behavior. However, the real system for studying such behavior remains scarce. Here, by combining magnetization measurements and first-principles calculations, we show that semi-ionic F (s-F)-doped graphene (s-FG) exhibits peculiar s-F-concentration-dependent magnetism and is a real platform for such study. Especially, with the decrease of the s-F concentration of s-FG, its ferromagnetic $T_{\mathrm{C}}$ exhibits an unusual increase. First-principles calculations show that the magnetism of s-FG (i) can be understood by the picture of RKKY behavior that the localized spins are mainly located on s-F adatoms accompanied by partial spin polarization of nearby carbon atoms, while (ii) it deviates the typical sublattice-dependent RKKY behavior presented as a nonmagnetic or antiferromagnetic state for localized spins on the same sublattice for some s-FG structures. The peculiar behavior is proposed to be related to the electron transfer effect, which can be evidenced by the good consistency of magnetism of s-FG by an effective Anderson impurity model.

The weak Mott insulating regime of the triangular lattice Hubbard model exhibits a rich magnetic phase diagram as a result of the ring exchange interaction in the spin Hamiltonian. These phases include the Kalmeyer-Laughlin type chiral spin liquid (CSL) and a valence bond solid (VBS). A natural question arises regarding the robustness of these phases in the presence of a weak spin-orbit coupling (SOC). In this study, we derive the effective spin model for the spin-orbit coupled triangular lattice Hubbard model in the weak Mott insulting regime, including all SOC-mediated spin-bilinear and ring-exchange interactions. We then construct a simplified spin model keeping only the most relevant SOC-mediated spin interactions. Using infinite density matrix renormalization group (iDMRG), we show that the CSL and VBS phases of the triangular lattice Hubbard model can be stabilized in the presence of a weak SOC. The stabilization results from a compensation between the Dzyaloshinskii-Moriya interaction and a SOC-mediated ring-exchange interaction. We also provide additional qualitative arguments to intuitively understand the compensation mechanism in the iDMRG quantum phase diagrams. This mechanism for stabilization can potentially be useful for the experimental realization of quantum spin liquids.

By employing determinant quantum Monte Carlo simulations, we investigate a checkerboard lattice with next-nearest-neighbor hopping $t^{\prime }$ as the frustration-control parameter, which exhibits an energetically partial flat band in the system. Our numerical simulation identifies the dominant pairing symmetry of the checkerboard lattice Hubbard model, and we reveal the competition between the $d$ wave and the $d+is$ wave in the parameter space of the electron filling $\langle n\rangle$ and the frustration control parameter $t^{\prime }/t$. To ensure the reliability and accuracy of our results, we evaluate the sign problem. We also find that the spin susceptibility, the effective pairing interactions of different pairing symmetries, and the superconducting instability are enhanced as the on-site Coulomb interaction increases, demonstrating that superconductivity is driven by strong electron-electron correlation. Our work provides a further understanding of pairing symmetry in the Hubbard model and improves prospects for exploring rich correlated behaviors in frustrated systems.

Nonlinear Hall effect (NHE) can be generated via Berry curvature dipole (BCD) on nonequilibrium Fermi surface in a nonmagnetic system without inversion symmetry. To achieve a large BCD, strong local Berry curvatures and their variation with respect to momentum are necessary, and hence, topological materials with strong interband coupling emerge as promising candidates. In this study, we propose a switchable and robust BCD in the newly discovered dual quantum spin Hall insulator (QSHI) ${\text{TaIrTe}}_4$ by applying out-of-plane electric fields. With an increasing electric field, a magnitude and sign switchable BCD could be found under different doping levels. Besides, topological phase transitions and the insulator-metal transition can also be observed in the primitive cell and the CDW phase, respectively. This work presents an instructive strategy for achieving a switchable and robust BCD within dual QSHIs, which should be helpful for designing the NHE-based THz radiations detector.

The photonic Chern insulator without time-reversal symmetry has received great attention in recent years because it possesses truly backscattering-immune one-way edge modes, which are robust against any local perturbations and play crucial roles in robustly controlled electromagnetic fields. However, the implementation of topological boundary channels necessitates a substantial number of trivial bulk domains, significantly limiting the utilization efficiency of topological photonic structures. In this work, we theoretically propose and numerically demonstrate the construction of photonic Chern insulators with yttrium-iron-garnet cylinders with hyperbolic lattices, which are regular tessellations in non-Euclidean space with constant negative curvature. In particular, two types of hyperbolic lattices, the dice and kagome hyperbolic lattices, are applied to design the photonic Chern insulators. Interestingly, we demonstrate that magnetic photonic crystals with hyperbolic lattice can support boundary-dominated one-way edge modes with nontrivial real-space Chern numbers even with significantly reduced bulk sites compared to that of traditional photonic Chern insulators. Our work extends photonic Chern insulators into the non-Euclidean space, and is anticipated to have potential applications in designing highly efficient photonic devices with strong robustness.

Rare-earth materials hold promise to realize exotic magnetic states owing to synergy between electron correlations and spin-orbit coupling. Recently, quasi-two-dimensional honeycomb magnets $A_2{\mathrm{PrO}}_3$ $(A=\text{alkali metals})$ were predicted to be good candidates for Kitaev quantum spin liquids, as the Kitaev-type bond-dependent anisotropic interactions dominate over bond-independent isotropic Heisenberg ones. However, experimental observations are negative, questioning the energy hierarchy in ${\mathrm{Pr}}^{4+}$ ions assumed in the conjecture on the basis of the conventional Russell-Saunders coupling scheme. Here, we revisit the exchange interactions in these Pr compounds, by explicitly taking into account the ionic states beyond the assumption. We show that, while increasing the octahedral crystal field splitting, which was assumed to be negligibly small, relative to the spin-orbit coupling, the Kitaev-type interactions are suppressed to be subdominant compared to the prevailing Heisenberg ones. Our finding compromises the contradiction as arising from the peculiar ionic state of the high valence ${\mathrm{Pr}}^{4+}$.

Recently, a novel collinear antiferromagnetic (AFM) phase termed altermagnet has been identified. It displays combined crystal and spin rotation symmetry, inducing significant spin splitting without spin-orbit coupling (SOC). This mechanism shifts the paradigm in spintronics, enabling AFMs to contribute to novel spintronic devices. However, previous work has mainly concentrated on bulk systems, with less attention given to two-dimensional (2D) materials due to their reduced dimensionality. To address this challenge and shed light on 2D materials, we have proposed a series of 2D altermagnets, namely, $\mathrm{V}{X}_4$ $(X=\mathrm{F}, \mathrm{Cl}, \mathrm{Br}, \mathrm{I})$, which feature an altermagnetic phase and substantial nonrelativistic spin splitting. Furthermore, we have explored their potential applications in the anomalous Hall effect (AHE). Our findings reveal that compared to other bulk materials, the anomalous Hall conductivities (AHC) of these 2D altermagnets are competitive. Moreover, the predicted AHE in these altermagnetic monolayers displays pronounced anisotropy, which allows for precise manipulation of electron spins via external fields. Our work provides insight into the potential of 2D materials in the exploration of altermagnets, paving an alternative way toward low-dimensional spintronic devices.

We report a full-potential (FP) screened spherical wave based tight-binding muffin-tin orbital (TB-MTO) method to implement the all-electron density functional theory (DFT). In this work, to realize accurate calculation of the full charge density, full potential, Hamiltonian, and total energy, a double augmentation scheme is introduced to represent the basis TB-MTO. To demonstrate the high accuracy of the FP-TB-MTO method, we apply it to compute the total energy and extract the equilibrium lattice constants, phase ordering, and elastic constants for bulk Co, Ni, and Fe, and our results match very well with another well-established FP method. The FP-TB-MTO-based DFT provides an accurate first-principles tight-binding electronic structure method for material simulations.

Local interlayer charge polarization of twisted bilayer hexagonal boron nitride (t2BN) is calculated and parametrized as a function of twist angle and perpendicular electric fields through tight-binding calculations on lattice relaxed geometries lattice relaxations tend to increase the bandwidth of the nearly flat bands, where widths smaller than $\sim 1$ meV are expected for $\theta \le 1.{08}^{\circ }$ for parallel BN/BN alignment, and for $\theta <1.5^{\circ }$, for the antiparallel BN/NB alignment. Local interlayer charge polarization maxima of $\sim 2.6$ pC/m corresponding to interlayer electron density differences of $\sim 1.3\times {10}^{12}{\mathrm{cm}}^{-2}$ are expected at the AB and BA stacking sites of BN/BN aligned t2BN in the long moire period limit for $\theta \ll 1^{\circ }$, and evolves nonmonotonically with a maximum of $\sim 3.5$ pC/m at $\theta =1.6^{\circ }$ before reaching $\sim 2$ pC/m for $\theta =6^{\circ }$. The electrostatic potential maxima due to the t2BN moiré patterns are overall enhanced by $\sim 20\%$ with respect to the rigid system assuming potential modulation depths of up to $\sim 300$ mV near its surface. In BN/BN aligned bilayers, the relative areas of the AB or BA local stacking regions can be expanded or reduced through a vertical electric field depending on its sign.

We investigate the transport properties, including Andreev reflection, $0\text{−}\pi$ transition, and crossed Andreev reflection, of graphene-based superconducting junctions modulated by an off-resonant circularly polarized light and a staggered sublattice potential. Using the Dirac-Bogoliubov–de Gennes equation and the Blonder-Tinkham-Klapwijk formula associated with numerical calculations, we can make the following findings. In a graphene-based normal conductor/superconductor (NS) junction, the differential conductance from Andreev reflection can be used to distinguish band structures. However, in a graphene-based superconductor/normal conductor/superconductor (SNS) junction, the valley polarization is induced and brings in a $0\text{−}\pi$ transition. Moreover, in a graphene-based normal conductor/superconductor/normal conductor (NSN) junction, the opposite valley polarization in two normal regions can lead to the pure and even perfect crossed Andreev reflections. The appreciable differential conductance of the pure crossed Andreev reflection is found for the 20–50 nm junction length, far less than the previous result in the graphene-based NSN junction, and it is helpful for the integration of superconducting quantum circuits. Our findings provide experimental possibilities to characterize the band structures, to realize the valley polarization-induced $0\text{−}\pi$ transition, and to design the valley-based highly efficient Cooper pair splitter in the light-modulated graphene-based superconducting junctions.

We study the many-body ground states of three-component quantum particles in two prototypical topological lattice models under strong intercomponent and intracomponent repulsions. At band filling $\nu =3/4$ for hard-core bosons, we demonstrate the emergence of the three-component fractional quantum Hall (FQH) effect characterized by the $\mathbf{K}$ matrix, through an exact diagonalization study of fourfold quasidegenerate ground states with a robust spectrum gap and the combined density-matrix renormalization group calculation of fractional drag charge pumping. Further, we formulate the topological characterization of FQH states of three-component Bose-Fermi mixtures at various fillings by the $\mathbf{K}$ matrix. Finally, we discuss the possible generalization of our approach to identify non-Abelian three-component spin-singlet FQH states.

Understanding the doped Mott insulator is a central challenge in condensed matter physics. In this work, we first explicitly identify a new sign structure in the $t\text{−}{t}^{\prime }\text{−}J$ model on the square lattice that replaces the conventional Fermi statistics for weakly interacting electrons. Then we show that the singular, i.e., the phase-string part of the sign structure in the partition function can be precisely turned off in a modified model. The density matrix renormalization group method is then employed to study these two models comparatively on finite-size systems, which is designed to unveil the consequences of the phase-string component. We find that the hole pairing is present not only in the quasi-long-range superconducting phase but also in the stripe phase of the $t\text{−}{t}^{\prime }\text{−}J$ model. However, once the phase-string is switched off, both the superconducting and stripe orders together with the underlying hole pairing disappear. The corresponding ground state reduces to a trivial Fermi-liquid-like state with small hole Fermi pockets that is decoupled from the antiferromagnetic spin background. It is in sharp contrast to the original $t\text{−}{t}^{\prime }\text{−}J$ model where large Fermi surfaces can be restored in the stripe phase found at $t^{\prime }/t<0$ or the superconducting phase at $t^{\prime }/t>0$ in the six-leg ladder calculation. Our study clearly demonstrates that the strong correlation effect in doped Mott insulator should be mainly attributed to the long-range quantum entanglement between the spin and charge, which is, nonperturbatively, beyond a simple spin-charge separation under the no double occupancy constraint.

We analyze theoretically the anisotropic optical conductivity of the $n$-doped type-II three-dimensional (3D) Dirac semimetal (DSM) ${\mathrm{PtTe}}_2$. With the effective Hamiltonian, which describes the anisotropic and tilted 3D Dirac cone in bulk ${\mathrm{PtTe}}_2$, the optical conductivities induced by the linearly polarized light are evaluated using the energy-balance equation derived from the Boltzmann equation. The in-plane optical conductivity ${\sigma }_{xx}\left(\omega \right)$ is similar to that of isotropic and nontilted Dirac systems, whereas a unique out-of-plane optical conductivity ${\sigma }_{zz}\left(\omega \right)$ has been found due to the tilt of the Dirac cone of ${\mathrm{PtTe}}_2$ along the $k_z$ direction. Both ${\sigma }_{xx}\left(\omega \right)$ and ${\sigma }_{zz}\left(\omega \right)$ are contributed by intraband and interband electronic transitions, where the interband transitions show more distinct anisotropic properties. We show that both ${\sigma }_{xx}\left(\omega \right)$ and ${\sigma }_{zz}\left(\omega \right)$ depend sensitively on energy relaxation times, temperature, and electron density, which enables their broad tunability in ${\mathrm{PtTe}}_2$, and promotes tailored applications of this and similar type-II 3D DSMs.

Altermagnetism is a newly predicted type of magnetism with spin-polarized electron bands but zero net magnetization. Here, we investigate the impurity scattering effect in an altermagnetic metal by using the $T\text{−}\mathrm{matrix}$ approach. We first calculate the impurity-induced local density of states (LDOS) in momentum space for a single scalar/magnetic impurity. In the absence (presence) of spin-orbit coupling and Zeeman field, the LDOS exhibits (does not exhibit) $C_4$ symmetry due to the $d\text{−}\mathrm{wave}$ nature of the Fermi surfaces. In real space, we obtain the analytical expressions for the Friedel oscillation of LDOS based on the $k·p$ Hamiltonian. The behavior of LDOS oscillations without spin-orbit coupling and Zeeman field also shows $C_4$ symmetry. The amplitude of the LDOS decreases with $r^{-1}$ in all directions at long distances, which is the same as that in conventional metals since they both have parabolic dispersion. However, the period of Friedel oscillations in an altermagnet is strongly anisotropic and depends on the direction of observation. Specifically, along the direction with (without) the spin splitting of the Fermi surface, the oscillation periods of the spin-up and spin-down LDOS are different (identical), resulting in an unfixed (a fixed) period at a given energy for the total LDOS. The difference between the two periods is proportional to the magnitude of the spin splitting of the Fermi surface. Our results reveal unique and universal features of the Fermi surfaces of an altermagnet that can be tested by spin-polarized scanning tunneling microscopy.